The roofs of coal mine shafts require support during a mining operation. This support is provided by roof bolts which are anchored into the rock strata found above the coal seam. In order to attach the roof bolts to the roof of a coal mine, many holes must be drilled into the rock strata and must be spaced closely enough to provide a strong, safe roof in the mine.
In the manufacture of prior art roof drilling tools, it has been the practice to make the drill body of a material such as steel and to mount an abrasion-resistant insert at the cutting end. The insert is formed of a hard material and is usually anchored in place in the body of the drill by soldering or brazing it in place.
Likewise, masonry drills have been known and are commonly used for drilling holes in especially hard, friable material such as masonry or stone. These drills are usually comprised of an elongated body or shank having a spiral groove or grooves formed along its length and having a diametrically extending straight groove on its leading end. A hard insert is set into the straight groove and is held in place by soldering or brazing.
The inserts described above usually have sharp cutting edges on the leading end so that the drills might be effectively used in the coal, hard masonry or stone material. The inserts must be capable of resisting wear, fracture, and the abrasive action of the chips from the material being drilled. Cemented carbides such as cobalt bonded tungsten carbide are at present the most commonly used materials for such drill bit inserts.
The speed with which holes can be drilled, the maintenance of this penetration rate and the wear resistance of the tools are important factors in such operations. Therefore, improvement in any of these factors is desirable, and has to some degree been achieved by changing the composition of the cemented carbide material, such as by adjusting the carbide to binder ratio, by selecting from various binder metals (e.g. Co or Ni-Fe), by adjusting the carbide grain size or by changing the insert geometry.
The coating of cemented carbide cutting tool inserts for metal removal applications with oxides, carbides, nitrides, and carbonitrides is known. However, prior to the present invention it has been widely accepted in the art that little or no improvement in performance could be expected for such coated drill bit inserts for mining or masonry applications. Conversely, it has been taught that such coating is contraindicated in mining applications (Colin M. Perrot, Ann. Rev. Mater. Sci. 9, 23 (1979) at p. 27). Although some coatings have been tried, for example boride coatings, the aluminum oxides, and the transition metal carbides, nitrides and carbonitrides have been considered not sufficiently hard for thin coatings on inserts for drilling in hard materials such as rock, coal and the like. (See for example U.S. Pat. No. 4,268,582 to Hale et al., column 1, lines 25 to 31.)
Thick polycrystalline layers of diamond or cubic boron nitride are commonly applied to tool bits for such uses as deep well drilling to provide the required wear resistance, but the thin wear resistant coatings described above have, again, not been considered sufficiently hard for such purposes.
U.S. Pat. Nos. 4,268,582, referenced above, and 4,343,865 disclose cemented carbide compacts for use in tools used for machining, rock drilling, and coal cutting, each having a boride coating such as titanium boride, hafnium boride, zirconium boride or tantalum boride. In the U.S. Pat. No. 4,268,582 patent, in interlayer of one or more layers of carbides, nitrides, or carbonitrides of groups IV B and V B elements provides improved bonding of the boride coating, but the increased wear resistance is provided by the outer boride coating. Similarly, in the U.S. Pat. No. 4,343,865 patent, a thin (about 1 micron) intermediate layer of a carbide, nitride, or carbonitride of a group IVB element, or a mixture thereof, prevents diffusion of elemental boron from a bonding layer underlying the boride coating into the substrate. In both of these patents, the boride outer layer is considered necessary to increase the wear resistance.